As schematically represented in FIG. 1, a continuous molten zinc plating apparatus comprises a plating bath (pot) 1, support roll 2, sink roll (pot roll) 3, sleeve 4, hanger (arm) 5 and a snout 6. In the apparatus shown in FIG. 1, a steel plate 7 supplied to the bath through the snout 6 is directed as shown by the arrow to the sink roll 3 supported by the hanger 5, and after being plated with a given amount of zinc, it is recovered from the bath 1 through the support roll 2. In the figure, the rolls rotate in the direction indicated by the arrow.
Conventionally, the structural members of the continuous molten zinc plating apparatus are produced by casting or plastic working of low-carbon steel or stainless steel (e.g. JIS SUS No. 304,309) or cast stainless steel (e.g. JIS HH). For example, cast stainless steel from which the support roll, sink roll, sleeve and bearing metal are produced is exposed to mechanical wear of fluid zinc and its resistance to molten zinc attack (hereunder referred to as erosion resistance) is so low that it must be replaced every one or two weeks. Therefore, researchers are making experiments on the build-up welding of Stellite alloy or plasma-spray coating of tungsten carbide (WC) on the surface of stainless steel.
Low-carbon steel of which the plating bath or snout is made has poor erosion resistance in molten zinc even when its degree of fluidity is relatively low. Therefore, the wall of the plating bath or snout is made of a thick plate or lined with a thin sheet of Hayne's alloy (e.g. HA-No. 25) generally considered to have relatively high erosion resistance in stationary zinc.
However, even the lined structural member does not present satisfactory erosion resistance when molten zinc is in a fluid state (e.g. the inside of the snout, or where the member is contacted by the topmost level of bath or in the vicinity thereof).
A support roll having good erosion resistance can be made of a structural member build-up welded with Stellite alloy since Stellite alloy has good mechnical wear resistance and exhibits relatively good erosion resistance in fluid zinc. On the other hand, Stellite alloy is so hard that the support roll may cause a steel plate being galvanized to develop a flaw and the support roll may develop a crack on the surface when it is being reground. Therefore, the structural member made by build-up welding with Stellite alloy cannot have a long useful life. Besides, as is typically found with Stellite No. 6 whose standard composition is Co: 61%, Cr: 28%, W: 5%, Ni: 1.5%, and others: 4.5% (by weight), Stellite alloys have a high content of expensive Co and this presents a high-cost problem.